The POSTal Numeric Encoding Technique (POSTNET) was developed by the U.S. Postal Service (USPS) to provide an optimized bar code system for encoding ZIP code information on letter mail. Postnet is a compact bar code format designed to be reliably read and decoded at high speeds by optical reading systems. The basic elements of the printed code are binary digits which are represented as full bars and half bars. Full bars represent "1"s and half bars represent "0"s. Each code character represents a decimal (base ten) numeric digit (0 through 9). The specification for postal bar code is contained in chapter 5 of Automation-Compatible Mail, DMM Issue 43, Jun. 21, 1992, available from the Postal Service, and Letter Mail Barcode Update, May, 1982, available from the Postal Service; both incorporated herein by reference.
Impact line printers comprise a continuously revolving type carrier having fully formed type elements such as alphanumeric characters moving past a row of individually operable print hammers. A control system, which might include a print line buffer for storing a line of data to be printed, a band image buffer which stores an image of the arrangement of the characters on the type carrier along with timing and hammer firing circuitry, selectively operates the hammers in synchronism with the motion of the characters. Imprints of the fully formed characters are produced on a print medium by causing the print medium and the type elements to be impacted against each other as a result of the selective operation of the print hammers while the type carrier is in motion. The print medium is incremented one or more line spaces at the completion of a print line and the process repeated for successive lines of print.
U.S. Pat. No. 3,656,426 by Potter and Austrian patent 236995 disclose line printers which simultaneously print a bar code under their corresponding alphanumeric characters. U.S. Pat. No. 5,056,429 by Hirosaki discloses a thermal printer in which symbol guards are printed around the bar code to prevent wrinkling in the vicinity of the bar code of the thermal transcription ink ribbon and resulting removal of ink. In the embodiment of FIG. 10 of '429 the guard consists of a rectangle surrounding the bar code. In Japanese citation JA 56-86758 to Tagusari smudging of small type area characters (e.g. 1) is decreased by adjusting the radius between the engraved characters and a print band on which they are formed.
Bar codes consisting of a mixture of narrow and wide bars have been successfully printed using impact line printers. In U.S. Pat. No. 5,007,748, narrow bars are placed together on one character and characters with wide bars are produced by overstriking so that the type elements have equal prints surface areas. IBM TDB Vol. 23, No. 10, March, 1981 and IBM TDB Vol. 26, No. 2, July, 1983 disclose multi-length bar codes. Japanese citation JA 58-114956 discloses dots which are separate from each character and which print to indicate when the character becomes worn.
The above citations are incorporated herein by reference.